High entropy alloys (HEAs) have been proposed for more than one decade, and it arouses a great deal of attention in metallic materials during recent years. Material researches have revealed that many HEAs exhibit excellent toughness, unique deformation behavior, and great radiation-resistance as promising metallic materials. However, different from traditional alloys, HEAs are with several principal alloying elements challenging the alloy design especially from thermodynamic calculations. Compared with the binary alloys, there is limited thermodynamic data in the central region of the phase diagram of multiple component alloys. For all that, we can still use the Calphad to attract useful information in the design of HEAs. Combining the experimental data, we have proved the validity of pseudo-binary phase diagrams in the multi-component alloys. Based on these results, we have demonstrated that the Calphad is used for the design of the FCC HEAs hardened with Mo, Ta and Nb alloying elements.